This invention relates to an electrostatic charged-particle analyzer.
In recent years, the technology of the surface analysis of a sample has been remarkably advanced. It is extensively carried out to determine the composition, the electronic structure, etc., of a sample by irradiating the sample with a primary beam, such as electron beam, ion beam or an X-ray beam, and by analyzing the energy of the charged particles such as Auger electrons, scattered ions and photoelectrons emitted from the surface of the sample.
In general, the emitted charged particles are of low energy. In order to enhance the analytical sensitivity, therefore, it is necessary to more efficiently detect the emitted charged particles. To this end, it is desirable to increase the ratio of the solid angle of charged particle rays entering a detector (accepted solid angle) relative to the entire solid angle of charged particle rays emitted from the sample surface in response to the irradiation thereby by the primary beam.
Examples of charged particle analyzers for providing an enhanced detecting efficiency which have hitherto been proposed are shown in FIGS. 1 and 2 in connection with an Auger electron spectrometer as an example.
In the construction of the prior-art charged-particle analyzer as shown in FIG. 1, numeral 1 designates an electron gun portion, numeral 2 a focusing and deflecting system for a primary electron beam, numeral 3 a sample, numeral 4 Auger electrons emitted from the sample by irradiation with the primary electron beam, numeral 5 a detector, and numeral 6 a cylindrical mirror type analyzer. A feature in the construction of this charged-particle analyzer having heretofore been used is that, since the axis of the primary electron beam and the axis of the cylindrical mirror type analyzer 6 are coincident, the detecting efficiency of the Auger electrons 4 emitted from the surface of the sample 3 is very high. Another advantage is that, since the electron gun portion 1 is disposed outside the cylindrical mirror type analyzer 6, the evacuating operation is easy.
The prior-art charged-particle analyzer shown in FIG. 1, however, is disadvantageous in that, since the focusing and deflecting system 2 is of the electromagnetic type and is disposed inside a cylindrical electrode of the cylindrical mirror type analyzer 6, the Auger electrons 4 are subject to the influence of a leakage magnetic field, resulting in a lowering of the energy resolution. Moreover, since the primary electron beam passes in close proximity to the detector 5, scattered electrons ascribable to the scattering of the primary electron beam occur, and some of them enter the detector 5 to cause a lowering of S/N (signal-to-noise) ratio.
On the other hand, in a structure wherein the electrostatic type of focusing and deflecting system is used, and wherein it is disposed inside the cylindrical electrode of the cylindrical mirror type analyzer 6 along with the electron gun portion 1, the areal resolution on the sample 3 is degraded. Especially in case where a field emission type electron gun is employed, the evacuation of the electron gun portion 1 becomes a problem.
The difficulties of the prior-art charged-particle analyzer shown in FIG. 1 are solved to some extent by another system, which is illustrated in FIG. 2. FIG. 2 is a constructional view of the charged-particle analyzer disclosed in Japanese Utility Model Laid-open Publication No. 15286/1975.
In the figure, numeral 1 designates an electron gun portion, numeral 2 a focusing and deflecting system for a primary electron beam, numeral 3 a sample, numeral 4 Auger electrons emitted from the sample by irradiation with the primary electron beam, numeral 5 a detector, numeral 7 a parallel plate type analyzer, numeral 8 a slit plate, and numeral 9 a deflecting and focusing system.
In this example, the Auger electrons 4 emitted from the sample 3 are analyzed by the parallel plate analyzer 7, and they are focused on a circumference about the axis of the primary electron beam. They pass through the slit plate 8 disposed at this position, and further advance outwards with respect to the axis. They are deflected towards the axis by the deflecting and focusing system 9, and are detected by the detector 5 disposed on the axis of the primary electron beam.
Owing to such construction, the electron gun portion 1, the focusing and deflecting system 2 and the parallel plate type analyzer 7 can be separately and individually formed. This solves the problem of the evacuation of the electron gun portion 1, the problem of the lowering of the energy resolution due to the leakage magnetic field, the problem of the lowering of the S/N ratio, and the problem of the degradation of the areal resolution on the sample surface.
In this case, however, considering the accepted solid angle, it is understood that a measurement at a high energy resolution is difficult. In the example of FIG. 2 of the charged-particle analyzer having heretofore been used, the accepted solid angle is determined by the effective solid angle of the parallel plate type analyzer 7.
In general, the ratio T of the accepted solid angle to the whole solid angle of the parallel plate type analyzer 7 is given by the following equation under the optimum conditions: EQU T = .OMEGA./.OMEGA..sub.o = 2 .alpha..sub.o sin 45.degree.
Here, .alpha..sub.o = .sqroot..DELTA./10 (where .DELTA. denotes the energy resolution). Assuming, for example, .DELTA. = 1 .times. 10.sup.-2, the value of T becomes: EQU T = 4.47 .times. 10.sup.-2
in the structure employing only the cylindrical mirror type analyzer as in the prior-art charged-particle analyzer illustrated in FIG. 1, the ratio T' of the accepted solid angle to the whole solid angle is T' = 10.32 .times. 10.sup.-2. As compared with this value, the value T of the structure of FIG. 2 is small. Accordingly, the performance of the example of FIG. 2 lowers considerably in this aspect.